Fate Determination of the Flavin Photoreceptions in the Cyanobacterial Blue Light Receptor TePixD (Tll0078)

PixD (Tll0078, Slr1694) is a BLUF (sensor of blue light using FAD)-type blue light receptor protein of the thermophilic cyanobacterium Thermosynechococcus elongatus BP-1 and the mesophilic cyanobacterium Synechocystis sp. PCC 6803. BLUF protein is known to show light-induced ∼10 nm red shift of flavin absorption that is coupled with strengthening of the hydrogen bond between the O(4) of the isoalloxazine ring and a certain amino acid residue. According to the 3D structure of TePixD we determined, O(4) of the ring is linked to Gln50 and Asn32. A survey of flavin-interacting residues by site-directed mutagenesis showed that Gln50 but not Asn32 is essential for the normal red-shifting photoreaction. Here, we further studied the role of Gln50 and its close neighbor Tyr8. All the mutated proteins of Gln50 and Tyr8 (Q50A, Q50N, Y8A and Y8F) lost the normal red-shifting photoreaction. Y8A, Y8F and Q50N, instead, showed a light-induced flavin triplet state and a low yield of subsequent flavin reduction that is analogous to the photocycle of the LOV (light-oxygen-voltage-sensing) domain of phototropins, while Q50A did not. Fourier-transform infrared (FT-IR) analysis of N32A showed that O(4) of the ring is hydrogen-bonded to Asn32 both in the light and dark. These results, together with the 3D structure, indicate that the hydrogen bond network of Tyr8-Gln50-O(4)/N(5) (flavin) is critical for the light reaction of the BLUF domain. Based on the structural and functional similarities of the BLUF and the LOV domain of phototropins, we propose that the interaction between apoprotein and N(5) of flavin determines the photoreaction of the flavin-binding sensors.